1. Field of the Invention
The present invention relates to a thin-film transistor (TFT) suitable for pixel display switching elements in an active matrix display scheme and to a method of producing the same.
2. Description of the Related Art
FIG. 1 is a cross sectional view illustrating the structure of a bottom gate type thin-film transistor. In fabrication of the thin film transistor, a gate electrode 2 of a high-melting point metal such as tungsten or chromium is formed on the surface of an insulating transparent substrate 1. Both sides of the gate electrode 2 are externally tapered on the transparent substrate 1. A silicon oxide film 4 is deposited via a silicon nitride film 3 on the transparent substrate 1 on which the gate electrode 2 is disposed. The silicon nitride film 3 blocks impurities contained in the transparent substrate 1 from intruding into an active region (to be described later). The silicon oxide film 4 works as a gate insulating film. A polycrystalline silicon film 5 is stacked on the silicon oxide film 4 to cross the gate electrode 2. The polycrystalline silicon film 5 acts as an active region.
A stopper 6 of an insulating material such as silicon oxide is formed on the polycrystalline silicon film 5. The polycrystalline silicon film 5 covered with the stopper 6 acts as a channel region 5c while the remaining polycrystalline silicon films 5 respectively act as a source region 5s and a drain region 5d. The silicon oxide film 7 and the silicon nitride film 8 are stacked on the polycrystalline silicon film 5 on which the stopper 6 is formed. Both the silicon oxide film 7 and the silicon nitride film 8 act as an interlayer insulating film for protecting the polycrystalline silicon film 5 including the source region 5c and the drain region 5d and for dispositing the drain line.
A contact hole 9 is formed in a predetermined position of the silicon oxide film 7 and the silicon nitride film 8 formed over the source region 5s while a contact hole 9 is formed in a predetermined position of the silicon oxide film 7 and the silicon nitride film 8 formed over the source region 5d. A source electrode 10s to be connected to the source region 5s is formed in the contact hole 9 while a source electrode 10d to be connected to the drain region 5d is formed in the contact hole 9. An acrylic resin layer 11 transparent to visible rays is stacked over the silicon nitride film 8 in which the source electrode 10s and the drain electrode 10d are formed. The rough surface caused by the gate electrode 2 and the stopper 6 is buried with the acrylic resin layer 11 to become a flat surface.
A contact hole 12 is formed in the acrylic resin film 11 on the source electrode 10s. An ITO (Indium Tin Oxide) transparent electrode 13 is connected to the source electrode 10s via the contact hole 12 and extends toward the surface of the acrylic resin layer 11. The transparent electrode 13 acts as a pixel electrode for the liquid crystal display panel.
A plurality of the above-mentioned thin-film transistors are disposed in a matrix form on the transparent substrate 1, together with pixel electrodes 13. Image data supplied to the drain electrodes 10d is input to the pixel electrodes in response to scanning control signals applied to the gate electrodes 2.
The polycrystalline silicon film 5 is preferably formed with polycrystalline silicon of sufficiently large grain size to act as an active region of a thin-film transistor. The excimer laser annealing method is known as a method of increasing the grain size of the polycrystalline silicon film 5. In this laser annealing method, an amorphous silicon is deposited on the silicon oxide film 4 acting as a gate insulating film. Hydrogen contained in the amorphous silicon film is expelled through a heating process at low temperatures. Then the silicon is temporarily melted by irradiating the excimer laser and is then recrystallized. Since a desired portion on the transparent substrate 1 is locally heated to a high temperature using the laser annealing method, a glass with a low melting point can be used as the transparent substrate 1.
Since the polycrystalline silicon film 5 crystallized through the laser annealing method has many crystalline defects, electrons moving therein tend to be easily trapped. Hence, the crystallized silicon is not desirable as the active region in the transistor. In order to solve such problems, an insulating film containing a great number of hydrogen atoms is formed on the polycrystalline silicon layer 5 temporarily formed. The crystalline defects are then buried with hydrogen atoms by annealing the insulating film in a nitrogen atmosphere.
A silicon nitride film is known as an insulating film containing a large number of hydrogen atoms. The hydrogen atom concentration of a silicon nitride film formed through the plasma CVD method is normally order of 1022/cm3 and is larger by two digits, compared with the hydrogen atom concentration (order of 1020/cm3) in the silicon oxide film formed through the plasma CVD method. For that reason, silicon nitride films are used as a hydrogen atom supply source.
Generally, since the silicon nitride film formed on the active region deteriorates the characteristics of a transistor, a silicon oxide film is formed between the active region and the silicon nitride film as shown in FIG. 1. However, the silicon oxide film 7 of a critical film thickness lying between the polycrystalline silicon film 5 and the silicon nitride film 8 may not supply sufficient hydrogen atoms into the polycrystalline silicon film 5. This problem means that a high-temperature annealing process or prolonged annealing time are required in fabrication steps, thus resulting in a decrease in productivity.
The present invention is made to solve the above-mentioned problems.
Moreover, the objective of the invention is to provide a thin-film transistor in which crystalline defects in a semiconductor film are effectively buried with hydrogen atoms to optimize the film thickness.
Another objective of the invention is to provide a method of manufacturing a thin-film transistor in which crystalline defects in a semiconductor film are effectively buried with hydrogen atoms to optimize the film thickness.
According to the present invention, a thin-film transistor comprises a gate electrode formed on a substrate; a gate insulating film formed on the gate electrode formed on the substrate; a semiconductor film formed on the gate insulating film; and an interlayer insulating film stacked on the semiconductor film; wherein the interlayer insulating film including a silicon oxide film in contact with the semiconductor film and a silicon nitride film in contact with the silicon oxide film; the thickness of the silicon oxide being set to a value of equal or less than (a thickness of the silicon nitridexc3x978000 xc3x85)xc2xd.
According to the present invention, the thin-film transistor further comprises a stopper region formed on a channel region of the semiconductor film; the total thickness of the stopper film and the silicon oxide being set to a value of equal or less than (a film thickness of the silicon nitridexc3x978000 xc3x85)xc2xd.
Moreover, according to the present invention, a thin-film transistor comprises a semiconductor film formed on a substrate; a gate insulating film formed over the semiconductor substrate; a gate electrode formed on the gate insulating film so as to cross to the semiconductor film; and an interlayer insulating film formed on the gate insulating film so as to cover the gate electrode; the interlayer insulating film including a silicon oxide film in contact with the semiconductor film and a silicon nitride film in contact with the silicon oxide film; the thickness of the silicon oxide being set to a value of equal or less than (a thickness of the silicon nitridexc3x978000 xc3x85)xc2xd.
In the thin-film transistor according to the present invention, the total thickness of the gate insulating film and the silicon oxide is set to a value of equal or less than (a thickness of the silicon nitridexc3x978000 xc3x85)xc2xd.
According to the present invention, a silicon oxide film and a silicon nitride film are formed as an interlayer insulating film on a semiconductor film acting as an active region. The silicon nitride film acts as a supply source for introducing hydrogen atoms into the semiconductor film. The silicon oxide film prevents the silicon nitride film from coming into contact with the semiconductor film. Since the thickness of the silicon oxide film is varied according to the thickness of the silicon nitride film, the silicon oxide film does not block hydrogen atoms introduced from the silicon nitride film into the semiconductor film.
In another aspect of the present invention, a thin-film transistor manufacturing method comprises a first step of forming a gate electrode on a major surface of a substrate; a second step of forming a gate insulating film on the substrate so as to cover the gate electrode and then forming a semiconductor film on the gate insulating film; a third step of forming an interlayer insulating film on the semiconductor film; and a fourth step of heating the semiconductor film and the interlayer insulating film at a predetermined temperature to introduce hydrogen atoms contained in the interlayer insulating film into said semiconductor film; wherein said third step including sub-steps of stacking a silicon oxide film in contact with said semiconductor film to a first film thickness, and stacking a silicon nitride film in contact with said silicon oxide film to a second film thickness, said first film thickness being set to a value of equal or less than (said second film thicknessxc3x978000 xc3x85)xc2xd.
Furthermore, according to the present invention, a thin-film transistor manufacturing method comprises a first step of forming a semiconductor film on a major surface of a substrate; a second step of forming a gate insulating film on the semiconductor film and forming the gate electrode on the gate insulating film so as to cross to the semiconductor film; a third step of forming an interlayer insulating film on the gate insulating film so as to cover the gate electrode; and a fourth step of heating the interlayer insulating film at a predetermined temperature to introduce hydrogen atoms contained in the interlayer insulating film into the semiconductor film; wherein, the third step including sub-steps of forming a silicon oxide film in contact with the semiconductor film to a first film thickness, and stacking a silicon nitride film in contact with the silicon oxide film to a second film thickness, the first film thickness being set to a value of equal or less than (the second film thicknessxc3x978000 xc3x85)xc2xd.
According to the present invention, after a silicon oxide film and a silicon nitride film are formed on a semiconductor film in the third step, the films are heated in the fourth step. Thus, hydrogen atoms contained in the silicon nitride film are introduced into the semiconductor film through the silicon oxide film. In this case, since the thickness of the silicon oxide film is varied according to the thickness of the silicon nitride film, hydrogen atoms contained in the silicon nitride film are sufficiently introduced into the semiconductor film without being blocked by the silicon oxide film.
In the thin-film transistor with the above-mentioned structure, even when a silicon nitride film is formed on the polycrystalline silicon film forming an active region via the silicon oxide film, the crystalline defects in the polycrystalline silicon film can be certainly terminated with hydrogen atoms supplied from the silicon nitride film. Therefore the requirements of the annealing process which is implemented to introduce hydrogen atoms from the silicon nitride film to the silicon oxide film can be relaxed. This feature simplifies the fabrication process, thus improving the manufacturing yield.